In the case where one desires to have a family of internal combustion engines starting with an engine block provided with a cylinder bore of a predetermined size and having the next engine provided with an engine block which has an increased cylinder bore size, the most opted for approach has been to continue use of the cylinder head for the original engine without changing the valve sizes. This approach inevitably results in the power output and fuel efficiency of the larger bore engine falling short of what both could have been had the valves been enlarged in proportion to the increase in bore size. An expensive alternative is to redesign the whole cylinder head. However, even that solution is not always possible because the tappet centers must change with respect to the cylinder centers with a corresponding change to the camshaft centers and to the cam gear diameters in geared double-overhead camshaft trains. Also, changes of this sort may not be possible due to space requirements dictated by cylinder head bolt sizes and location and under-hood clearances.
An even worse situation exists if it becomes desirable to start with an engine having a cylinder bore of a predetermined size and having the next engine provided with a cylinder bore which is to be reduced in size. In such case, only two possibilities exit. The first is to redesign the entire cylinder head with smaller valves. This is not only very expensive, as mentioned above but, sometimes, is extremely difficult because the camshaft and the tappet bores crowd the spark plug or fuel injector. Moreover, reducing the tappet diameter as a possible solution results in a reduction in the valve lift or the camshaft diameter which may not be acceptable.
The second possibility is to reduce the valve size in proportion to the bore size reduction without moving the valve center relative to the center of the cylinder. This solution, however, would be unacceptable to an engine designer because it would result in the power output of the engine suffering a major reduction due to the fact that each valve has its diameter reduced at least by the same numerical (not proportional) amount that the cylinder bore diameter is reduced.